FIG. 4 shows a prior art semiconductor laser device for a laser printer. In FIG. 4, reference numeral 1 designates a laser chip for a laser printer. Facet protection films 2 comprising, for example, Al.sub.2 O.sub.3 , are produced at the front and rear laser facets of the laser chip. A metal wire 4 is connected to the laser chip 1. The laser chip 1 is mounted on a sub-mount 5 and the sub-mount 5 is disposed on a heat sink 6. An output laser light beam 7 is emitted from the front facet of the laser chip 1 and a monitor laser light beam 8 is emitted from the rear facet of the laser chip 1.
FIG. 5 shows a semiconductor laser used for the device. In FIG. 5, reference numeral 11 designates a p-type GaAs substrate. A p-type AlGaAs cladding layer 12 is disposed on the p-type GaAs substrate 11. An n-type GaAs current blocking layer 13 is disposed on the p-type AlGaAs cladding layer 12. A p-type AlGaAs cladding layer 14 is disposed on the n-type GaAs current blocking layer 13. An undoped AlGaAs active layer 15 is disposed on the p-type AlGaAs cladding layer 14. An n-type AlGaAs cladding layer 16 is disposed on the undoped active layer 15. An n-type GaAs contact layer 17 is disposed on the n-type AlGaAs cladding layer 16. A p-side electrode 18 is disposed on the p-type GaAs substrate 11 and an n-side electrode 19 is disposed on the n-type GaAs contact layer 17.
When a current larger than a threshold current flows through the semiconductor laser 1, laser oscillation occurs and laser light beams are emitted from both of the front and rear facets of the laser chip. The laser beam 8 emitted from the rear facet is utilized as a monitor light. Facet protection films 2 comprising, for example, Al.sub.2 0.sub.3 , are provided at the laser light emission facets in order to prevent oxidation of the facets. When the facet is oxidized, light absorption at the facet surfaces increases, generating heat due to the laser light absorption at the surface. The crystal in the vicinity of the facet may melt thereby destroying the laser device. Herein, the reflectivity of the facet is about 30 % regardless of the existence of the protection film 2.
Generally, a semiconductor laser used in a laser printer is pulsed. In this case, the laser light output gradually decreases with the passage of time during each pulse, as shown in FIG. 6. In FIG. 6, reference characters (P.sub.A) and (P.sub.B) respectively represent laser light output direct after the start of laser oscillation and directly before the end of laser oscillation and reference character (t) represents the time period during which the laser is switched on.
When the laser is switched on and off at a constant operating current as described above, the laser light output transiently decreases. The degree of the transient decrease in the laser light output during pulsed operation is represented as a droop. The droop is defined in the following equation, referring to FIG. 6. ##EQU1##
Because the printing density of the laser printer is determined uniquely by the laser light output, when the droop .DELTA.P is large, printing intensity variations are generated. Accordingly, it is necessary to reduce the droop .DELTA.P in order to obtain a printing result having no variation. In the semiconductor laser for 9 laser printer having a construction as shown in FIG. 1, however, the droop .DELTA.P is larger than 10 %, and there is a possibility that printing variations will occur with that value of droop.